1. Field of the Invention
The present invention relates to desmodromic valve and cam systems for internal combustion engines configured with overhead cams. In particular, the present invention relates to a cam system which eliminates the springs found in conventional valve systems by implementing a design which utilizes cam lobe assemblies with internal follower grooves in combination with a follower rocker arm and hydraulic lifters. The present invention also relates to camshafts which have replaceable cam lobes providing various duration/lift adjustability options.
2. Background of the Invention
Most conventional internal combustion piston driven engines utilize valve trains to induct an air/fuel mixture into the cylinders and to expel the burned air/fuel mixture from the cylinders. Typically, each cylinder is assigned at least one poppet intake valve and at least one exhaust poppet valve. The valves are typically pushed down by rockers thereby opening the valve. In a conventional pushrod engine, the other end of the rocker is in contact with one end of a pushrod. Further, the other end of the pushrod is typically in contact with a lifter which is in contact with a camshaft lobe. In overhead cam configurations, the other end of the rocker typically is in direct contact with the camshaft lobe, thereby eliminating the need for pushrods. To close the valve, that is to pull the valve back up so that it seats, most conventional valve trains utilize a spring which concentrically surrounds the valve stem. When the valve stem is pushed down to open the valve, the spring is compresses. The valve is closed when the spring decompresses thereby pulling the valve stem up through the valve guides until the head of the valve seats in the valve seat.
For example, in a typical four-stroke engine, an intake valve is opened by a rocker which receives an input from a cam lobe while the piston goes down inducting an air/fuel mixture into the cylinder (i.e., induction stroke). While the valve stem is being pushed down through a valve guide, a spring concentrically positioned around the valve stem is compressed. Next, when the piston moves upward, the intake valve is pushed back up through the valve guide when the spring decompresses. At this stage in the combustion process, the air/fuel mixture is compressed (i.e., compression stroke). With both valves closed so that the combustion chamber is sealed tight, a spark is then produced by a spark plug which ignites the air/fuel mixture wherein the rapidly expanding hot gases force the piston downward with great energy creating power (i.e., power stroke). The exhaust valve then opens as the piston moves back up it expels the burned air/fuel mixture (i.e., exhaust stroke).
The aforementioned conventionally configured valve train systems for opening and closing the valves has proven to be highly effective and reliable in the past. However, closing the valve by the force of the spring does have some disadvantages. Most notably, pushing the valves open against the force of the springs consumes engine power. The springs in an engine induce considerable tension into the valve train because they continuously force the valve mechanism against the rocker as the camshaft rotates. In other words, the valve springs are continuously pushing the valves closed. Another disadvantage is that because the cam mechanism cannot afford to have any “bounce” from the springs, the cam profile has to be somewhat gentle, i.e., it must gently push the valve, but never shove it. This means the valve must open slowly like a water faucet—not quickly like a light switch, for example. Another disadvantage is that when the motor is turned at high RPM's, the valves can “float” and hit the piston. Valve float happens when the speed of the engine is too great for the valve springs to handle. As a result, the valves will often stay open and/or “bounce” on their seats.
To overcome these disadvantages, innovative desmodromic valve trains have evolved over about the last century; however, in a very slow technological pace and in most applications with little or limited success. The term “desmodromic” arises from the two Greek words: “desmos” (controlled or linked), and “dromos” (course or track). A desmodromic system is also known as system that provides “positive valve actuation” wherein both strokes are “controlled”. In other words, desmodromic valves are those which are positively closed by a leverage system or follower, rather than relying on the more conventional springs to close the valves. Typically, a desmodromic valve operating system utilizes a camshaft that controls both the opening and closing of the valve.
Desmodromic valve trains have several advantages over conventional spring closed valves trains. A first major advantage is that in a desmodromic valve system there is almost no wasted energy in driving the valve train. In other words, the constant force that the springs exert on the valve train is removed. Another advantage is that because there is no tension and no possibility of “bounce” in the desmodromic system, the cam profiles can be as steep as the engine designer wishes them to be. This desirable aspect allows the engine to be more powerful and more flexible. Thus, the manufacturer can use more radical cam grinds or profiles for better performance. Another advantage is that when the motor is turned at high RPM's or even over-revved, the valves are still controlled, whereas when the valves are returned by springs the valves sometimes can “float” and hit the piston.
Nevertheless, even though desmodromic valve trains have the aforementioned advantages, they have had limited success in large scale commercial applications due to reliability issues, complexity of design, and valve train binding to name a few reasons. For instance, one of the major disadvantages of desmodromic valve trains is their sensitivity to change in size of the separate component of the system. In particular, the individual components (valves, cam lobes, rockers, etc.) of the valve train become enlarged at elevated temperatures because of thermal expansion of the metallic components. Also, the components of the valve train wear, thereby, decreasing the size of the components. As a cumulative result, of both cyclic expansion and contraction of the components caused by heating, and the shortening of components caused by wear, the tolerances of the valve train can change. The end result, are components such as valves which do not seat properly, or unwanted binding in the valve train. Therefore, one of the major difficulties of prior art desmodromic valve train systems is the critical and accurate adjustment of various working components to ensure that the components operate together as intended without being subjected to binding, tension or excessive friction which results from the change of size in the individual components.
One species of desmodromic valve trains which has evolved in an attempt to solve the aforementioned problems includes desmodromic valve trains which utilize lash adjustors or hydraulic lifters to compensate for changes in size of the components of the valve train. Hydraulic lifters use the engine's oiling system to automatically adjust valve lash (clearance) to zero. Due to their dampening capabilities, hydraulic lifters help to eliminate any lash or binding problems on the system.
For instance, U.S. Pat. No. 3,430,614, entitled “Desmodromic Drive Arrangement,” to MEACHAM on Mar. 4, 1969, discloses a desmodromic system which utilizes a dashpot apparatus 88 (see FIG. 8) to compensate for changing tolerances and sizes of parts in a desmodromic system. In particular, MEACHAM provides an engine valve arrangement 24 in which the valve 26, 40 is positively opened and positively closed. The mechanism for closing the valve 26, 40 comprises a rocker arm 62, 64, a movable fulcrum structure 54, 56 arranged to bias the rocker 62, 64 toward a camming mechanism 66 on order to bind the camming mechanism 66, rocker arm 62, 64 and valve 26, 40 together to effectively operate as a unit. The spring means and dashpot apparatus 88 is arranged to produce a resistance to movement away from the camming mechanism 66 that is proportional to engine speed. Although the MEACHAM system appears to viable, one of the disadvantages of MEACHAM is that it requires a unique head 10. Further, the camshaft 66 is complex and the system requires two rockers per valve.
Another reference, U.S. Pat. No. 6,487,997, entitled “Springless Poppet Valve System”, issued to PALUMBO on Dec. 3, 2002, discloses a springless poppet valve system. The system includes a poppet valve 12 moveable between an open and closed position. The system includes an open cam and close cam. An open rocker arm 22 is provided which engages the open cam. The open rocker arm is operatively connected to the poppet valve so as to move the valve from the closed to open position. A close rocker arm 22 is provided which engages the close cam. The close rocker arm 22 is operatively connected to the poppet valve 12 so as to move the valve from the open position to the close position. An open hydraulic lifter 58 is pivotally connected to the open rocker arm. A close hydraulic lifter 60 is pivotally connected to the close rocker arm 22. Although the PALUMBO system successfully incorporates hydraulic lifters to increase reliability and reduce maintenance, it uses two lifters and two rockers per valve which increases the expense of the system. Furthermore, the PALUMBO system appears to require a special head design.
Also, Japanese Patent No. JP60081410, entitled “Compulsorily Valve Opening and Closing Apparatus for Internal-Combustion Engine,” issued to JIYUNJI et al. on May 9, 1985, discloses a system a motorcycle head and valve train which utilizes an “automatic hydraulic slit adjusting apparatus” to compensate for changing tolerances and sizes of parts in desmodromic systems. In particular, JIYUNJI discloses a rocker arm 49 for compulsorily opening a valve and a rocker arm 50 for closing the valve, which are installed in swingable ways onto an eccentric shaft 54 supported onto a rocker-arm shaft, and driven by a valve opening cam 46 or a valve closing cam 47. The eccentric shaft 54 has an arm 71 which contacts an automatic hydraulic slit adjusting apparatus 51, and the eccentric shaft 54 is turned by the extension and contraction of the automatic slit adjusting apparatus, and the slit can be automatically adjusted by shifting each swingable-shaft core of the rocker arms 49 and 50 in the direction of each axis center of the valves 32 and 33. Although, it appears JIYUNJI provides a desmodromic system with enhanced reliability by incorporating a hydraulic slit adjusting device, the JIYUNJI system is configured only for motorcycle engines and is not easily adapted to conventional engines for automobiles.
Another prior art reference, U.S. Pat. No. 6,311,659, entitled “Desmodromic Cam Driven Variable Valve Timing Mechanism,” issued to PIERIK, on Nov. 6, 2001, discloses a system which utilizes stationary hydraulic lash adjusters 56 to compensate for changing tolerances and sizes of parts in desmodromic systems. In particular, PIERIK discloses a desmodromic cam driven variable valve timing (VVT) mechanism 10 which includes dual rotary opening and closing cams 18, 20 for actuating a rocker mechanism 34 that drives valve actuating oscillating cams. The dual rotary cam drive positively actuates the rocker mechanism 34 in both valve opening and valve closing directions, and thus, avoids the need to provide return springs to bias the mechanisms toward a closed valve position. It is noted that although the PIERIK invention is claimed to be “desmodromic”, it still utilizes valve springs (not shown; see col 3, lines 7–9), which are conventionally provided for biasing the valves in a closing direction. Therefore, even though the system is claimed to be “desmodromic” it appears that the inclusion of the valve springs still induce a binding tension to the valve train, which as an end result reduces the engine's power and efficiency. Furthermore, the VVT mechanism in the PIERIK invention is a complex system requiring numerous parts which does not yet have a proven track record with regard to reliability. Additionally, the PIERIK invention appears to require a unique head design which increases the expense of the overall system installation.
By utilizing lash adjustors or hydraulic lifters some of the disadvantages that have long been associated with desmodromic valve trains have been alleviated as taught by MEACHAM, PALUMBO, JIYUNJI and PIERIK. However, although MEACHAM, PALUMBO, JIYUNJI and PIERIK teach functional desmodromic systems, they still have similar disadvantages. One of the problems with the aforementioned desmodromic valve train systems is that they have not been adapted to be installed or “retrofit” into existing modern conventional engines. That is to say, the aforementioned prior art desmodromic systems appear to utilize specialized head designs which requires a unique head. Thus, to use the aforementioned desmodromic systems, the entire head and valve train must be replaced.
Therefore, it would be advantageous to provide a desmodromic valve and cam system which utilizes hydraulic lifters or the like that may be either integrated into a new engine design or of which may be retrofit onto an existing engine head design without requiring the head or valves to be replaced. Such a springless system would operate more efficiently than conventional valvetrains since the tension is reduced from the valvetrain resulting in greater horsepower and fuel economy; while the incorporation of hydraulic lifters or the like will make the desmodromic system more reliable. By providing a retrofit desmodromic system, the cost of the upgrade could be maintained lower than that of a system which requires the entire head to be replaced. It would further be advantageous to provide a desmodromic valve and cam system which is simple to manufacture, inexpensive and of which may be easily retrofitted into existing head designs which may have already been manufactured and of which are being currently sold. Furthermore, it would be desirable to provide a desmodromic valve and cam system which would have interchangeable cam lobes such that the cam duration/lift could be adjusted. With such a feature, various cam lobes having varying profiles, durations, lift, etc. could be utilized on the same system by merely replacing the cam lobes. Such features would provide a wide array of adjustability in regards to being able to tune the engine's performance characteristics.